| A novel scanning SQUID-on-tip (SOT) microscope has been designed and built. SOTs as small as 46 nm were fabricated at the apex of a sharp pulled quartz tip. The SOTs present flux noise down to 50n&PHgr; 0/Hz1/2 which is on-par with the best reported SQUIDs. They can operate at magnetic fields of up to 1 T and have a wide bandwidth, from dc to over 1 MHz. Because the SOTs have an ideal geometry for scanning microscopy, they may be scanned merely nanometers away from the surface of a sample. These properties present an unprecedented combination of resolution and sensitivity, and result in spin noise down to 0.38mu B/Hz1/2, almost two orders of magnitude better than any SQUID before.;The scanning SOT microscope is a versatile tool which can be used to investigate a wide variety of systems that display magnetic structure on the nm scale. The design of the microscope, fabrication and characterization of the SOT's, as well as examples from studies of flux flow in superconductors and emerging magnetic phenomena at the interface of oxides are presented in part I of the thesis.;In part II, I report using the scanning SOT microscope for the investigation of the dynamics of quantized magnetic vortices and their pinning by materials defects in lead films with unprecedented sub-A sensitivity to vortex displacement. We measured, for the first time, the fundamental dependence of the elementary pinning force of multiple defects on the vortex displacement, revealing a far more complex behavior than has previously been recognized, including striking spring softening and broken-spring depinning, as well as spontaneous hysteretic switching between cellular vortex trajectories. Our results indicate the importance of thermal fluctuations even at 4.2 K and of the vital role of ripples in the pinning potential, giving new insights into the mechanisms of electromagnetic response of superconductors. |
